The present invention relates to an apparatus and method for applying a coating to at least a portion of a substrate having desired surface properties. In particular, the invention relates to a method for producing polymer coatings with various morphologies on medical implants, such as stents.
Coatings are often applied to medical implants, such as pacemakers, vascular grafts, catheters, stents, heart valves, tissues or sensors to have desired effects and increase their effectiveness. These coatings may deliver a therapeutic agent to the lumen that reduces smooth muscle tissue proliferation or restenosis and may comprise a polymer carrier. Furthermore, implants may be coated to improve surface properties such as lubriciousness, to achieve enhanced biocompatibility and to control the timing and rate of release of the therapeutic agent being delivered. Balloon delivery systems, stent grafts and expandable stents are specific examples of implants that may be coated and inserted within the body. Stents such as described in U.S. Pat. No. 4,733,665 are tiny, expandable mesh tubes supporting the inner walls of a lumen used to restore adequate blood flow to the heart and other organs.
Such coatings have been often applied to the surface of an implant by spray coating. An atomizing device including an orifice and an internal fluid passage leading to said orifice is typically placed perpendicular to the longitudinal axis of the substrate to be coated. The droplets 52 generated by the atomizing device are expelled through the orifice and the majority of the droplets hit the surface of substrate 54 at an impact angle of approximately θ=90 degrees with a comparatively high impulse force resulting in a dense coating splat 53 as shown in FIG. 1A.
The comparatively high coating compaction of the produced coating may however result in an inhomogeneous coating thickness and cracks. In addition, it may be difficult to produce porous coatings, which can be used in medical implants as reservoirs for the retention of therapeutic agents and may be desirable to enhance tissue ingrowth and tissue healing.
Also, conventional coating methods, such as described in U.S. Pat. No. 6,143,370, may not allow the coating of a stent so that the morphology of the coating layer can be changed instantaneously, which may be desirable to accommodate the need for different elution profiles as may be required by the medical application.